It is an urgent task to seek other energetic resources or different energy conversion pathways to replace the burning of fossil fuels such as gasoline or diesel, due to the increasing worldwide energy demand and environmental concerns. One of the promising efforts is the development of fuel cell technology. Fuel cells exhibit exciting performance advantages for power generation by converting the chemical energy of a fuel directly into electricity. The intense interest in fuel cell technology stems from the fact that fuel cells are environmentally benign and extremely efficient. Among various types of fuel cells, the proton-exchange membrane fuel cells (PEMFCs) and direct methanol fuel cells (DMFCs) are appealing for automotive and portable electronic applications owing to their low operation temperatures.[1-3] Unfortunately, the slow rate of the oxygen reduction reaction (ORR) on the PEM and the high cost of Pt electrocatalyst still remain serious limitations to many applications. In this regard, it is rather challenging to explore more active and low-cost catalysts superior to the standard carbon-supported platinum (Pt/C) particle systems which are traditionally employed.
Precious metal, platinum (Pt), is traditionally used as a high-performance electrocatalyst for proton-exchange membrane fuel cells and fine chemical synthesis. Due to the high-cost and the scarcity of Pt, it is an urgent task to develop substitutes for the pure Pt-catalyst. To date, one of the most successful accomplishments is to partially substitute Pt using less expensive 3d-transition metals. It was also realized that the shape and surface structure of nanocrystals (NCs) play a significant role in electrocatalytic activity and reaction durability. For instance, it has been reported that cubic Pt NCs possess unusual catalytic activity in oxidation reactions. As well-known, the electron density of state is actually sensitive with the surface structure, and different crystal facets could have diverse catalytic natures.
Platinum nanoparticles (Pt NPs) have been extensively studied because of their unique catalytic properties in various significant applications.[1-8] It has been shown that the catalytic activity of Pt NPs is highly dependent on the surface atomic arrangements of the particles.[9-11] For example, previous studies on oxygen reduction in adsorbing acidic solutions show that Pt {100} planes are more active than Pt{111} planes,[12, 13] and the current density measured on Pt nanocubes is higher than that of truncated cubic Pt NCs.[14] Pt nanocubes[15-17] have therefore received more attention as electrocatalysts than other morphologies such as multipods[18, 19] and one-dimensional nanostructures.[20, 21] To further reduce the overall use of expensive Pt and afford the potential of poisoning resistance, platinum-based bimetallic Nanocrystals such as Pt—Ni,[22, 23] Pt—Co,[24-28] and Pt—Cu[29-32] have attracted increasing interest. Moreover, recent reports indicate that electrocatalytic activities of some platinum-containing bimetallic Nanocrystals are superior to those of pure Pt metal.[25-28]
Both PEMFCs and DMFCs use polymer electrolyte membrane (PEM) and platinum (Pt)[4, 5] or Pt-based alloys catalysts. Unfortunately, the slow rate of the oxygen-reduction reaction (ORR), the high cost and the vulnerability toward reaction poisons of Pt electrocatalyst remain serious limitations to many applications.[3, 6] In this regard, it is crucial for fuel cell development to explore more active and poison-resistant catalysts that are superior to the traditionally employed carbon-supported platinum (Pt/C) particle systems. There has been considerable progress on the search for Pt-based bimetallic electrocatalysts, such as forming Pt—Pd nanocomposites,[7, 8] or Pt-monolayer on a second metal,[9] or alloying Pt with less expensive 3d-transition metals,[10, 11] including Fe,[11, 12] Co,[13, 14] Ni,[15, 16] Cu,[17, 18] Cr,[12] and Mn.[15] It was reported that the catalytic activity of Pt3M (M=V, Ti, Co, Fe, Ni) is significantly improved[19, 20] with strong resistance to poisonous substances.[21] Recently, Stamenkovic et al. demonstrated that extended single crystal surfaces of Pt3Ni {111} exhibit an enhanced ORR activity that is 10-fold higher than Pt{111} and 90-fold higher than the current state-of-the-art Pt/C catalysts.[22] Such a remarkable activity was attributed to the weaker OH adsorption arising from the decrease of the d-band center on the Pt-skin formed by surface segregation. Like many other heterogeneous catalysis studies,[23, 24] a fundamental question is whether such a high activity observed on the extended single crystal surfaces can be obtained from nanometer-sized particles. To bridge this size gap, the challenge is to produce crystal facet-controlled monodisperse {111}-bounded Pt3Ni NCs. Although Monte Carlo simulation suggested that {111}-facet-terminated Pt3Ni nanoctahedra would be energetically stable and have a surface segregation profile similar to that of the extended Pt3Ni surfaces,[22, 25] direct experimental evidence has not been obtained.